A typical ratchet-type tensioner includes a plunger slidable in a plunger-accommodating hole in a tensioner housing. The plunger is movable along an advancing and setback direction, and is biased in its advancing, i.e., protruding, direction. The plunger either abuts a flexible transmission medium such as a timing chain directly, or applies tension to the transmission medium through a device such as a pivoted tensioner lever on which the transmission medium slides.
A typical prior art ratchet-type tensioner 500 is disclosed in Japanese Utility Model No. 2559664 and illustrated in FIG. 9. In the tensioner, a plunger 514 is slidable in a plunger-accommodating hole in a housing 512, and protrudes from the housing, being biased in the advancing direction by a spring 518 and by oil pressure within a chamber 516 formed by the plunger and the housing.
A piston 526 slides in the housing 512 in a direction orthogonal to the direction in which the plunger 514 slides. An oil sub-chamber 520 is formed by the piston 526 and the housing 512, and an oil passage 544 supplies oil under pressure to the oil sub-chamber 520, urging the piston away from the plunger 514. A spring 534 biases the piston 526 toward plunger, opposing the force exerted by the oil in sub-chamber 520. Spring 534 is located within an air chamber 528 on the side of piston 526 opposite from the sub-chamber 520. An air hole 532 in communication with the air chamber 528 is closable by a rod 524, to which the piston 526 is attached, when the piston 526 is moved away from the plunger against the biasing force exerted by spring 534 by hydraulic pressure in sub-chamber 520 and the piston 526 moves against a biasing force of the second spring 534.
A rack of teeth 538 is provided on plunger 514, and a plurality of teeth 536, capable of engaging the rack teeth 538, is provided at the end of rod 524 opposite form the end that is arranged to close off air hole 532. Surfaces of teeth 536 and 538 for blocking retraction of the plunger are formed at a right angle to the direction in which the plunger 514 moves.
FIG. 10 shows another known ratchet-type tensioner 550, having a plunger 551 movable within a housing and urged by a biasing force in the advance direction applied by a plunger-biasing spring and by oil within a high-pressure oil chamber (not shown) to apply tension to a chain. A ratchet mechanism 560, prevents the plunger 551 from being set back by a reaction force generated in the traveling chain in the setback direction. The ratchet mechanism 560 includes a ratchet element 561 received in the housing and slidable in a direction orthogonal to the advancing/setback direction of the plunger 551. Rack teeth 552 provided on the plunger 551 are engageable by ratchet teeth 562, and a ratchet-biasing spring 566 biases the ratchet element 561 in an engaging direction along the direction of sliding movement of the ratchet element so that the ratchet teeth 562 engage the rack teeth 552.
In the tensioner shown in FIG. 10, the maximum backlash corresponds to the pitch P5 of the rack teeth. Rattling noise is generated due to abutment of the rack teeth with the ratchet teeth as the plunger advances and sets back, and flapping of the chain also occurs, generating additional noise.
A greater backlash results in greater rattling and flapping noises. Reducing the inclination angle α5 of the sliding surfaces 553 of the rack teeth 552 as shown in FIG. 10 is one way to reduce backlash and resulting noises. However, if the inclination angle α5 is reduced, the pitch becomes smaller, as shown by the single dot broken lines in FIG. 10, where the pitch is reduced to pitch P5a. When the plunger advances, the frequency of abutment of the rack teeth 552a with ratchet teeth, which will then have a tooth shape corresponding to the rack teeth 552a, increases. However, due to increased friction between the rack teeth and the ratchet teeth, the plunger can no longer advance as quickly, and cannot follow variations in tension in the chain. Thus, the application of tension to the chain is retarded.
The inclination angle α5 can be reduced without decreasing of the pitch P5 of the rack teeth 552, by increasing the depth of the rack teeth 552b to a tooth depth H5b, as shown in FIG. 10 by two-dot broken lines. Increasing the tooth depth, however, generates other problems. Movement of the ratchet element in the sliding direction increases, and fluctuations in the ratchet biasing force exerted by the ratchet biasing spring 566 increase. The result is destabilization of the behavior of the plunger, and increase of the friction between the rack teeth 552b and ratchet teeth, which will then have a tooth shape corresponding to that of the rack teeth 552b. 
When the plunger advances rapidly, the ratchet element, being driven by the advancing plunger also recedes rapidly in the disengaging direction. In such a case, after the ratchet teeth disengage the rack teeth, the start of the return movement of the ratchet element in the engaging direction can be retarded due to the inertia of the ratchet element. When inertial retardation of movement of the ratchet element occurs in a ratchet mechanism such as mechanism 560, having rack tooth stop surfaces 554 that are parallel to the sliding direction of the ratchet element, the ratchet teeth 562 can jump over the rack teeth 552, and may not engage with all of the rack teeth.
Furthermore, when the plunger sets back, it is possible that the ratchet teeth will ride over the rack teeth so that the plunger continues to set back, even beyond its backlash distance, depending on the relative magnitude of the chain reaction force and the plunger biasing force. In that case, the rattling noise generated when the ratchet teeth are abutted by the rack teeth increases, and the flapping noise of the chain also increases.
This excessive setting back of the plunger, as the ratchet teeth ride over the rack teeth, occurs on starting an engine when there is not enough hydraulic pressure in the high-pressure oil chamber to restricting setback of the plunger, i.e., when the advance-direction biasing force is too small. When oil in the high pressure oil chamber of the tensioner is supplied by the engine oil pump, low hydraulic pressure typically occurs as the result of infiltration of air into the high-pressure oil chamber while the engine has been stopped for an extended interval of time.
There is also a case in which the plunger advances excessively due to looseness or elongation of the chain resulting from fluctuation in tension, or thermal expansion of the engine block or of the chain caused by changes in the temperature of the engine. When the plunger protrudes excessively, the chain is ultimately placed under excessive tension. And, if the plunger is restricted from setting back due to the reaction force from the chain when the chain is under excessive tension, the chain travels under excessive load, and generates noise.
The invention aims at solving the above problems by providing a ratchet-type tensioner in which the plunger responds quickly to changes in tension, in which the durability of the ratchet mechanism is improved, and in which noise caused by backlash is reduced. The invention also aims at providing a ratchet-type tensioner in which the behavior of the plunger is stabilized. The responsiveness of the plunger, the durability of the ratchet mechanism, the reduction in backlash noise, and stabilization of the plunger behavior, are addressed by a novel shape of the sliding surfaces of the rack teeth of the ratchet mechanism.
The invention also aims at providing a ratchet-type tensioner that prevents ratchet teeth from jumping over the rack teeth due to inertial retardation of the ratchet element when the plunger advances.
Furthermore, the invention aims at providing a ratchet-type tensioner capable of reducing noise caused by the setback of the plunger, and of reducing the load in, and the noise generated by, the chain or other transmission medium when under excessive tension.